Production of cement clinker



s. GOTTLIEB PRODUCTION oF CEMENT lCLINKER Feb.'17, 1970 Filed sept, 25, 19:68

MGE

United States Patent U.S. Cl. 263-53 14 Claims ABSTRACT F THE DISCLOSURE A method of cement manufacture in vertical kilns. The coal is ground with water to slurry to be within a predetermined fineness range and blended with raw meal the iineness of which is related to the neness of the coal slurry. This produces raw meal layers aroung the coal particles to protect the volatile matter of the coal from premature escape and the coal particles themselves from premature combustion.

This application is a continuation-in-part of my application Ser. No. 425,614 led Jan. 14, 1965, now abandoned, entitled Prouction of Cement Clinker.

This invention relates to the production of cement clinker in vertical kilns and more particularly relates to a method of fuel application to vertical type kilns to irnprove the performance thereof.

A vertical kiln consists of a circular shaft of about 28-30 feet height, 8-9 feet diameter, which may extend at the top to l0 feet. The kiln is lined with rebricks and insulated against undue heat loss. In normal operation it is kept full of material. Nodules in the top zone, sintered clinker below. In the normal operation of such a kiln to produce cement clinker the -raw material, which must have the right chemical composition and be ground to a specified degree of iineness with the necessary quantity of fuel admixed, is fed to a nodulizer of the type such as is shown in Australian Patent No. 152,109 granted June 30, 1953. The nodules so produced are fed into a rotating device, uniformly distributing them into the top part of the kiln. Here the nodules move downward against a counter-current stream of air, getting hotter and hotter until the temperature is suicient to ignite the fuel particles embedded The combustion of these particles maintain a floating hot zone in the top part of the kiln. Discharge of the sintered clinker is continuous, through a rotating grate and hydraulically or mechanically operated air lock system, while air is blown in underneath the grate.

It is understood from the above description of the vertical kiln manufacturing method as it operates today, that there must necessarily be a limitation, restricting the process to the use of coals with no or very low volatile content, as the volatile matter escapes above the hot zone where there is a shortage of oxygen, which can cause some of the volatile combustibles to escape from the kiln unburnt. According to Australian Patent No. 223,738 of Aug. 27, 1959 air is introduced into the kiln top to ignite the volatile matter, and while this can be achieved sucessfully, it was found that reducing the volume of the hot zone by the quite voluminous cone, means also reduction of kiln-output.

The object of the present invention is to approach the problem from a different angle, not by burning the volatiles higher up in the kiln with secondary air, but by protecting the volatiles from premature escape until they 3,495,811 Patented Feb. 17, 1970 ICC reach further down into the kiln, to ignite them with the normal air supply from below.

A further object of the present invention is to provide a process wherein the fuel is ground into a slurry with Water, the slurry is then blended with iinely divided cement making raw materials and the mixture is formed into nodules.

In a preferred form of the invention coating or lubricating the nodules to make them roll freely in the nodulizer without impediment and thus to form nodules of substantially equal size, and almost perfectly spherical in shape, is effected by feeding two streams of raw meal into the nodulizer; one, the major stream into the center after being blended with slurry and the other, the minor stream, without admixed slurry to one side of the pan. The center major stream with admixed slurry will form the nodules in the pan, while the side Ininor stream will coat them, i.e., lubricate them with dry powder so as to assist in causing them to become spherical and also to flow easily. To facilitate the formation of the nodules without sticking to the nodulizer-pan, a rotating type scraper may be provided, operating either in the same d1'- rection as the rotation of the nodulizer, or in the opposite direction dependent upon the chemical and physical characteristics of the raw materials.

The new process which could be termed slurry preparation for fine vertical kiln fuel has the following advantages:

(1) Possibility of using high volatile fuels in the vertical kiln with eliiciency and automation greater than hitherto feasible,

(2) Possibility of achieving further improved quality of product and higher output of clinker.

As illustration of how the invention can be put into practice, coal is accurately proportioned lo a suitable grinding mill where it is ground upon addition of Water into a coal slurry. This slurry is pumped into the kiln house and accurately proportioned into a blending conveyor, equipped with mixing paddles. The pulverized raw material is also proportioned into the same conveyor. For a kiln output of 200 tons of clinker per day, a paddle conveyor of about 14 feet long, 2 feet 6 inches diameter, r.p.m. is adequate. The paddle conveyor homogenizes the raw meal/coal slurry blend into an earth-moist mass of material which is then fed to the nodulizer.

As 12 feet length of the double action paddle conveyor mixer is sutlicient to homogenize the coal slurry with the dry raw meal into a uniformly homogenous earth moist mix, to be fed onto the nodulizer, someI raw meal can be by-passed to the nodulizer 4before the coal slurry is added. This dry meal can serve for the coating of the nodules, or for the nodule lubrication process as described above.

By the method as described every fuel particle, due to the minute water layer attracted to its surface, will be covered by a relatively dense layer of the tine raw meal, which would increase its density as drying progresses in the pre-heating zone of the kiln.

The well-known chemical reaction between the raw meal and the coal will become more pronounced due to the above described ymethod of coal-slurry preparation and nodule formation. This reaction, viz:

commences at lower temperature than that necessary for the dissociation of the carbonates (at 900 C.) viz:

CaCO3==CaO+CO2 thus before the layer of raw meal will become porous due to the above dissociation. Below 900 C. the raw meal layer which had adhered to the wet fuel particle quite strongly, also insulates it, protecting the volatiles from premature escape, changing the whole preheating curve. The escape of volatiles now occurs in a zone in the kiln, where the temperature has reached the level necessary for ignition of volatile matter.

In order to clarify the steps of the new process there is submitted a drawing in which:

FIGURE l is a diagrammatic simplified illustration of the coal slurry operation with the vertical kiln omitted.

FIGURE 2 is a greatly enlarged individual coal particle as secured in the first half of the paddle conveyor and FIGURE 3 is a similar view of an individual coal particle as secured in the second half of the paddle conveyor and entry into the nodulizer.

Quarry crushed limestone, coal and gypsum, as well as other ingredients if ne-cessary, are dumped into a ground level hopper and conveyed to storage silos, designed to conformity with the relevant raw material characteristics. Wet clay is discharged into a separate hopper and run through a comminutor to break up lumps, dried if necessary, before proportioning to the limestone. Alternatively, wet and sticky clay can also be processed without primary drying, by feeding it with a hoist-bucket on every truck of limestone entering the plant, to produce high and low lime blends with one or two buckets fed on each limestone truck.

The high and low limestone/clay blends, iron ore and other ingredients if necessary, are proportioned to the raw grinding mill which grinds and dries simultaneously, to produce raw meal of suitable lineness.

Reference is made to FIGURE 1 of the drawing wherein the process is schematically illustrated.

`Coal is fed into a wet grinding unit, indicated generally at 1, to be ground to coal slurry, which is precisely proportioned for a special design paddle mixer 2, into which also the raw meal is accurately fed from the weighing scale 3. The earth-moist product of the paddle mixer drops into a pan-nodulizer 4 which is equipped with rotating and stationary scraping devices (not shown), to produce nodules (pellets) of uniform composition, shape and size.

From the wet-mill 1 the coal slurry is first fed to the stirrer as shown and then by means of the pump 5 fed through the pipe 6 to the paddle conveyor 2. The slurry is fed into the paddle conveyor 2 at a point approximately one foot from the point where the raw meal is fed into the paddle conveyor from the weighing scale 3. The major portion of the raw meal and slurry is passed to the conditioning conveyor which leads the earth-moist product of the paddle mixer through the conveyor 7 to the pan-nodulizer 4. A minor quantity of the raw meal is fed through the conveyor 8 to the lower portion of the pan nodulizer 4 in order to lubricate the nodules. From the lower end 9 of the nodulizer 4 the nodules are discharged in the direction shown by the arrow into the top of the vertical kiln, not shown.

The control means for the water, coal, raw meal and slurry is indicated generally at 10 and such control means will be provided with control lights for indicating the slurry level and also with means for controlling and adjusting the percent of water supply.

Referring to FIGURE 2, the coal particle is indicated at 11 with a water layer 12 held by the coal surrounded by raw material particles 13. This indicates the general condition of an individual coal particle as developed in the first half of the paddle conveyor 2 which is Stage I.

In FIGURE 3 Stage II shows the condition of an individual coal particle in the second half of the paddle conveyor 2 wherein the coal particle 14 is covered with a moist raw material paste and particles.

Agitated by a push-button controlled variable speed rotating grate, situated at the bottom of the main body of the reactor, which is a vertical kiln shaft (not shown), the nodules moving downwards pass through several phases of preheating, decarbonizing, sintering and cooling. It is essential that the nodules should remain stable during all phases. Air is blown in below the grate by a constant air supply blower. As the air is moving countercurrent to the nodules, hot combustion gases with a high CO2 content, as generated in the lower parts of the hot Zone, may pass through temperature zones, in which the reaction CO2+C=2CO (proved by Boudouard) progresses at a relatively high rate. This would seriously affect heat economy of the kiln and also produce undesirable Fe Fe reduction, unless the coal particles are effectively protected. Application of the coal slurry method gives such greatly improved protection of the coal particles against excessive Boudouard reaction, as at a certain energy input during the wet grinding of coal, water will cover the particle surfaces with greater force than ordinary adsorption, to exert less vapor pressure than pure water at the same temperature.

Due to the relatively strongly adsorbed water on the surface of the coal particles, raw meal layers are formed which become dense and good insulators in the drying phase. The very desirable reaction (assumed and later proved by Hauenschild) is also enhanced in the coal slurry reactor, as the time during which the intimately contacting coal/raw material surfaces are protected, would be extended and thus the dissociation temperature of the CaCO3 (about 900 C.) reached somewhat later. The advantages as presented by high coal reactivity, would be much better utilized with coal slurry preparation, than by intergrinding coal and raw `material in the dry process.

Production routine of a coal slurry reactor is smooth and uneventful and thus the process is eminently suitable for full automation. The lower part of the reactor is a most efficient clinker cooler and air preheater.

The clinker is discharged by the grate continuously and at a uniform steady rate, by a hydraulic triple bucket system, maintaining pressure inside the kiln at constant level. The agglomerated sintered nodules of clinker are conveyed by vibrating conveyor and possibly dragchain, to a suitable crusher and then into storage.

The reactor shell is heavily insulated to ensure efficient heat economy. Fuel consumption, depending on many factors such as chemical and physical characteristics, net heat value, reactivity of coal, and also the fineness and physical characteristics of the raw meal, varies between 1450-2000 B.t.u. per lb. of clinker. The coal slurry process enables to operate the reactor with a much wider range of coals than hitherto possible in vertical kilns, including also coals with relatively high content of volatile matter.

The coal slurry reactor operates with much less dust emission than vertical kilns of earlier design. The type of dust collectors employed in a new plant must be based on a thorough study of materials and other local conditions.

Clinker and gypsum are proportioned to a cement grinding mill, operating in closed circuit. The finished cement is conveyed into silo storage and loaded into bulk carriers.

All cements, types I-V, can be manufactured in a coal slurry reactor, in full conformity with ASTM Standards, with strength specifications fulfilled far above the level of the requirements.

In coal/water systems, under certain conditions, an emulsifying eect leads to a relatively stable suspension, thus confirming that coal can absorb water with greater capillary surface binding force. When fine coal and lwater are stirred together, there is no emulsifying effect whatsoever: the coal will settle out relatively quickly of its suspended state, which was produced by stirring, leaving clear water above the sediment.

However, if the stirring is intensified, as can be achieved for instance in a tube mill, then at a certain energy input level the consistency and appearance of the coal/water suspension will suddenly change: it will obtain a uniformly black color as the surfaces of the coal particles will hold water with greater force than ordinary adsorption, exerting less vapor pressure than pure water at the same temperature. This produces a real suspended state and a very slow settlement rate.

With water held strongly on the surfaces of the coal particles, blending between the coal slurry and the pulverized raw meal, under closely controlled neness relations and proportionng conditions, will produce strongly adhering raw meal layers around every coal particle. The structure of the pellets produced under these conditions (A) in the pan nodulizer becomes very different in comparison to pellets produced under conventional conditions in a nodulizer as described in Australian Patent 152,109 of May 3, 1950, where the dry, pulverized blend of raw pulverized blend of raw material and coal is moistened by fine water spray on the nodulizer (B), a procedure which became standard practice during the past l years, having been adopted by Spohu and others. Extensive investigation proved that after identical drying procedures of nodules produced as per (A) and (B), porosity and plasticity were much higher with (A) than with (B). This difference is of very great practical signicance for kiln operation, as the different nature of the solid/fluid structure, achieved by coal slurry application, permits water to escape during drying procedures without disrupting the nodule.

One of the -most common setbacks of modern vertical kiln operation is the instability of nodules in the preheating lone of the kiln: Certain raw materials showed the tendency of forming nodules of such high density, that they cannot stand up to the internal pressures which build up inside the nodule during drying, thus causing their disintegration in the pre-heating zone. As soon as this happens, orderly sintering procedure, i.e. eicient and automated kiln operation is no longer possible. With the improving of nodule stability as achieved by the coal slurryapplication, modern vertical kiln technology has been placed on a -much sounder foundation, permitting now complete automation of the vertical kiln process by creating simple and stable conditions for pre-heating, calcining and sintering, which follow strictly pre-determined pattern without any possibility for changing conditions.

If the iineness of the coal and raw meal are accurately adjusted in accordance with preliminary test studies of the raw materials, then it is possible to achieve conditions under 'which the coal particles will be completely covered by moist raw meal layers which will initially be wettest on the contacting coal surfaces and on drying also the densest, while porosity of the drying raw meal layers will increase with greater distance from the surface of the coal particle, thus permitting water vapor and gases to escape without disruption. This produces most desirable conditions, mainly by extending the time during which the coal particles are protected during the preheating of the nodules, by the improved thermal insulation of the drying raw meal layers around individual coal particles, which permits their movement to a lower level in the kiln before the relevant temperature is vreached at which:

(a) the bulk of coal volatiles would escape (b) the coal particles themselves would ignite.

The extended time gained by the above protection of the coal particles also shortens the period during which temperature conditions for appreciable Boudouard reaction rate CO2+C=2CO prevail. This reaction always presented the greatest problem to vertical kiln technology because of counter-current ilow of air vs. coal, in contrast to ymost rings, where coal and air are supplied in parallel,

A great number of ideas and applications attempted to solve this problem by varying means and successes: One application was introducing secondary air from the top of the kiln in counter-current flow to the main air, and another method was to produce two different raw meals,one by grinding measured amounts of coal, limestone, clay, etc. together, and blending and storing this coal containing black meal in silos, the other by grinding limestone and clay, etc. separately without coal to white meal and applying this 'white meal for the coating of nodules which were produced from black meal with a water spray applied in a pan nodulizer. The coating of the black meal nodules by Lwhite meal is carried out in another pelletizer under closely controlled conditions in regard to the quantities of black and white meal proportions, and to the thickness of the white meal coat surrounding the black meal nodules.

The requirements for accuracy on the white mea coating are so great, that no successful application has proved feasible yet on such method. If the amount of coal in the black meal core nodule is slightly higher than calculated, adverse reducing reactions may take place producing clinker minerals containing ferrous ions, which oxidize slowly and disrupt the sintered clinker by their changed volume even by delayed action in storage. On the other hand, if the amount of coal in the black meal core nodules is slightly lower than calculated, sintering may become incomplete, with too porous, lightweight clinker produced, which has unacceptable free lime content and thus may lead to unsoundness. Although a conical, stepped grate to discharge such clinker smoothly was designated, without causing uneven movements of the material in the kiln, the remedy did not solve the problem of the qualitative shortcomings of clinker, emerging from any slight downward variation of coal content in the black meal core nodule.

There is of course, nothing in the black meal process which could have any influence on the stability of the nodules, as the water admixed with the raw meal, containing coal in pulverized form, only on the nodulizer, would not give sucient time for the coal to adsorb the water in a way as achieved by coal slurry preparation.

Apart from the points raised above, which make the fundamental difference between protecting the coal particles by the slurrying of coal in this application, and -coating of specially prepared nodules with white meal, there are fundamental differences also in regard to the Hauenschild reaction CaCO3-j-C=2CO|-Ca0. Rather than to control this reaction by accurately adjusting nodule size, white meal coating and thus core diameter and coal content of core, the Hauenschild reaction can now simply be kept'under control by adjusting the size of the coal particles which in the coal slurry application are always completely surrounded by meal layers, thuscontaining suicient CaCO3 for the reaction to proceed without harmful reducing processes.

The size of the coal particles in the coal slurry method depends on the physical characteristics of the coal as well as of the raw materials. As mentioned above, when coal and water are fed into a tube mill, a sudden change in slurry appearance and consistency will take place at a certain energy input, representing a stage at which water will be held on the surface of the coal particles with forces somewhat greater than normal adsorption. As soon as this stage is reached, no further coal grinding is necessary, thus milling conditions, grinding media load, mill throughput must be adjusted to meet these conditions. It is evident that the neness requirements of the coal in the coal slurry process are not only limited fby maximum size, but also by minimum size. The fneness requirements of the raw meal depend mainly on the physical characteristics of the raw materials: e.g. a raw meal manufactured from softish, porous limestone and sludgeable clay will break down easier with water when coal slurry is admixed to it, than hard and brittle raw materials which thus would require much finer grinding for the production of an acceptable coating power around the coal particles. An extensive investigation of the raw materials is required to determine the finenesses of coal slurry and the raw meal and the relevant equipment must be designed and operated accordingly.

The fineness of the coal slurry should preferably be 'within the range of 10 to 40% residue on 170 mesh sieve.

10 to 40% residue on 170 mesh sieve indicates the wide range of finenesses commencing with a coal, of which 10% is larger than the openings of 170 mesh sieve and 90% is smaller, to the other extreme of fineness where 40% is larger and 60% is smaller.

170 mesh sieve is a standard sieve with openings which would let particle sizes up to 88 microns pass.

The fineness of the raw meal should preferably fall within the range of 4 to 15% residue on 170 mesh sieve.

There is nothing new in mixing coal and raw materials with Water for kiln feed, but the present process involves the production of coal slurry under controlled conditions as above, for admixture in a paddle conveyor to produce an earth-moist product which is fed into a pan nodulizer. This process eliminates the necessity of separate milling equipment for the grinding of black and white meals, and separate black and white meal silos with relevant conveying and homogenizing equipment and separate nodulizers.

The sole objective of surface dusting in the present process is to make the nodules more perfectly round in shape to assure their unimpeded fiow through the feeding device into the kiln, and to assure a more uniform air flow.

An example of one application among many of the present invention is as follows.

Preliminary investigation of the raw materials revealed the following.

Raw materials prepared in the required proportions to produce a raw meal of the following chemical composition:

SiOZ, 13%

R203, 6.5% (R=Al and Fe) CaO, 45%

CO2 and traces of MgO-Ti02, 35.5%

The raw meal was formed into nodule and tested as set forth below with the following results:

Porosity test, 31.5% pore volume Water adsorption and resorption tests, 8-3.5; 6-2.5; 4 2 (first figures representing loss of water at certain temperature levels, second figures adsorption (resorption) of water after cooling back to room temperature) Hardness and wear, 25%/20 mins., 20 min. shaking as rubbing on motorized screen Water adsorption for normal nodule consistency, 17%

The coal was tested as set forth below.

The particle sizes in the coal slurry should be 120i60 microns. The particle sizes of the raw meal should be at 60130 microns.

It is essential that the preheating, decarbonizing, sintering and cooling process in the vertical kiln should proceed at a uniform rate. If this is maintained, the temperature of the discharged clinker will not vary by more than i50 C., and, in fact, operation of a kiln over an extended period of several months could be proved at such rate of uniformity.

Nearly constant temperature of discharged clinker also means the same degree of sintering, i.e a grate speed variations of not more than 20%.

To maintain the above described conditions, the following is essential:

(A) Uniform chemical composition and fineness- CaCO3 content should not vary by more than i0.2% to give a variation in the lime saturation factor of the clinker of not more than i2% The fineness required to maintain uniform sintering would only be assessed after a thorough study of the raw materials. Milling system which would not produce unnecessarily too much fines, while on the other hand, would eliminate oversize is preferred.

(B) Nodules-They must be of uniform shape, of sufcient resistance to be fed into the kiln without breakage, as well as of high stability to withstand the disrupting forces of steam and gas escape in the zones over the sintering section.

An elaborate testing procedure must be carried out to determine as to whether the materials conform with the above requirements. Among such tests are:

(a) Plasticity tests a determination of water requirement to reach a certain plastic consistency (b) Water adsorption and resorption tests on laboratory prepared nodules at 3 temperature levels: 220 F., 450 F. and 1000 F.

(c) Mechanical resistance and wear tests of nodules. lf this is not the case, then additives can usually solve the problem. Thus, it can be said that there are only very few materials which would be classified as totally unsuitable. However, in practice some materials may be regarded unsuitable notwithstanding the possibility that additives may permit the production of stable nodules, as the cost of such additives may be too high. In regard to coal, a wide range of tests must be carried out. Apart from B.t.u. value, moisture, ash and volatile constituents, it is essential to do work on coal reactivity as well. The method introduced by the Coal Research Laboratory, Carnegie Institute of Technology (reported by Mayers in Chemistry of Coal Utilization, vol. I, p. 897, John Wiley & Sons, Inc., New York, 1945) based on the crossing point determination, characterizes coals and cokes by the ternperature at which the reaction rates become great enough to produce fixed rates of temperature rise. Two values of this quantity, T15 and T75, define both the magnitude of the reaction rate and given temperature, as well as its rate of change with temperature. Values of these quantities, observed by Mayers (Industrial Engineering Chemistry, 291118-1937):

For the vertical kiln it is desirable to have a T15 figure not lower than 200 and not higher than 400.

Coal reactivity tests using CaCO3 of standard neness are also necessary: these are carried out in a molybdenum wire furnace (H2 gas flowing around the wires) with a simple titration test to check on the carbonate in a temperature range of 400 C.-850' C. It is also essential to determine the characteristics of volatile escape of the coal and temperatures at which such escape occurs and the approximate quantities. The B.t.u. value of the volatile gas must be calculated.

Coal slurry tests are also essential to determine the amount of Water at which pumpable slurry can be produced. This slurry, blended with the raw meal must produce a consistency which is not sufficient yet to form nodules, to enable either water to be added in the nodulizer or to increase water content in the coal slurry.

Variations and modifications may be made in the conduct of the method of the invention and in the arrangement of apparatus.

I claim:

1. A method of cement manufacture in vertical kilns, in which the coal is ground with water to slurry to be within a predetermined fineness range, then said slurry is blended with raw meal whose fineness has a predetermined ratio to the fineness of said coal slurry to produce raw meal layers around the coal particles to protect the volatile matter of the coal from premature escape and the coal particles themselves from premature combustion.

2. A method as claimed in claim 1 in which the coal and water are accurately proportioned by precision weighing devices into a ball mill having one compartment loaded with grinding balls of the same dia-meter, to produce the particle size distribution for the coal in the required range.

3. A -method as claimed in claim 2 in which the coal slurry is discharged by gravity flow into a container equipped with a stirring device and self-adjusting level indicator which is electrically interlocked with the coal and water feeding device to the mill.

4. A method as claimed in claim 3 in which the coal is proportioned by a flow meter into a paddle conveyor with the feeding point of the coal slurry situated at a distance of one foot from the entry point of the raw meal into said paddle conveyor.

5. A method as claimed in claim 1 in which the raw meal is accurately proportioned by a precision weighing device into a paddle conveyor.

6. A method as claimed in claim 1 in which the homogenizing of the coal slurry with the raw meal is eflected in a shaft paddle mixing conveyor.

7. A method as claimed in claim 1 in which a minor amount of the raw meal already weighed in a precision weighing device, is taken out of a paddle conveyor before the feeding point of the coal slurry, and fed to the nodulizer in a separate stream from the major amount fed to said nodulizer to lubricate the nodules for easier movement and to provide them with cleaner and rounder surfaces.

8. A method of cement manufacture in vertical kilns comprising grinding coal to a predetermined fineness tgether with water, to produce a free flowing uniform slurry, then homogenizing said slurry with a measured quantity of cement raw meal having a predetermined proportion to said slurry and then feeding the resulting earthmoist mix into a nodulizer.

9. A method as claimed in claim 8 wherein a double shaft paddlemixing conveyor effects said homogenizing.

10. A method as claimed in claim 8 wherein a minor amount of said cement raw meal is fed to said nodnlizer in a separate stream from the major amount fed to said nodulizer, said minor amount lubricating the nodules formed by said nodulizer.

11. A method according to claim 10 wherein said minor amount of raw meal is taken out ot a first section of said paddle conveyor before admixture of the coal-slurry, to perform said lubricating of said nodules.

12. A method as set forth in claim 1 wherein said coal is ground to be within the range of 10 to 40% residue on a mesh sieve.

13. A method as set forth in claim 1 wherein the neness of said raw meal is within the range of 4 to 15% residue on a 170 mesh sieve.

14. A method as set forth in claim 1 wherein said coal is ground to be within the range of 10 to 40% residue on a 170 mesh sieve and the iineness of said raw meal is within the range of 4 to 15% residue on a 170 mesh sieve.

References Cited UNITED STATES PATENTS 2,119,615 6/1938 Wendeborn 263-53 2,776,828 1/1957 Marcellus et al 263-53 3,305,611 2/1967 Spohn 263-53 JOHN J. CAMBY, Primary Examiner 

